This invention relates generally to a power transfer mechanism adapted to be incorporated in a multiple path drive system, for example a four-wheel drive vehicle. More particularly, it relates to a torque transfer case adapted to receive torque from a prime mover and to provide torque for one drive axle where two-wheel drive is adequate, and for both drive axles where four-wheel drive is required.
In recent years there have been many improvements in automotive drive trains, including improvements relating to the transfer of torque from a prime mover to drive axles. Where four-wheel drive systems are used, transfer cases have been developed which generally provide torque transfer directly to one axle and selectively to another axle. Some such transfer cases have included an overrunning mechanism which automatically engages and disengages the four-wheel drive function. This automatic feature is desirable. However, when the system is in coast, and in particular when the vehicle is proceeding down a grade which could not be negotiated safely by a conventional two-wheel drive vehicle, engine braking is delivered only to the rear wheels in the automatic mode of operation. It is now known that under these conditions it is advantageous to have the capability of effecting a locked-up mode of operation in which the transfer case output shafts do not rotate with respect to each other. In this locked-up mode, a driver could rely on engine braking delivered to both front and rear wheels. Thus there remains a need to provide an improved torque transfer case having an overrunning mechanism which in one mode engages and disengages automatically, and in another mode provides positive engagement.
Accordingly, it is an object of this invention to meet the continuing need and desire in the art for improvements in a torque transfer mechanism which normally provides torque transfer to one output shaft for establishing a two-wheel drive condition, and which engages through an overrunning device to provide torque transfer to two output shafts for establishing a four-wheel drive condition. The mechanism includes a unique construction to condition the overrunning device to provide either automatic four-wheel drive or locked-up four-wheel drive, as desired by the operator.